The beverage container manufacturing industry has, for many years, sought to reduce the weight and amount of metal used in two piece beverage cans. A manifestation of this trend is cans having tapered necks. There are two basic techniques used to transform a roughly cylindrical drawn and ironed can body into a semi-finished container with a tapered neck: spin necking and die necking. Spin necking involves rotating the cylindrical container about its centerline axis and applying pressure to the open end to reduce its diameter. Die necking involves moving the open end of the can over a series of progressive dies that gradually reduce its diameter while providing a tapered smooth surface.
A conventional die necking operation utilizes a stationary, circular die that surrounds a longitudinally translatable center ring (also referred to as a punch or a support ring). At the beginning of each necking stage, an exposed, accessible lower end of the center ring is stationary in an initially extended position relative to the stationary die until a pusher mechanism pushes a can (open end first) toward the die and over the center ring. After the can engages the center ring, the center ring and can retract within the die, which remains stationary, to form a neck shape of the can. The can and center ring reverse direction, and the can is disengaged from the die and center ring to constitute one stage of the necking operation. Conventional necking operations employ several necking stages.
For examples of die necking, U.S. Pat. No. 3,983,729 (Traczyk) discloses a die necking apparatus that also creates a flange on the end of the container that is used to attach the closure to the body. U.S. Pat. No. 4,774,839 (Caleffi et al.) generally describes a die necking operation wherein a series of turrets are used to transfer the container to each stage. Each turret has a die having a slightly different diameter and degree of taper so that the end diameter and shape of the neck are progressively reduced. Many considerations that go into the determination of the die shapes for each stage are well known, such as those illustrated in U.S. Pat. No. 5,355,710 (Diekhoff). Each of the three patents listed above are incorporated herein by reference in their entireties.
The necking tooling, specifically the center ring, may be driven upward either by a constant motion cam or a differential motion cam. A constant motion cam retracts (that is, moves upward within the die), the center ring at a rate that matches the rate at which the container moves (both relative to the stationary die) such that, after the container engages the center ring, there is no differential longitudinal movement between the center ring and the can. A differential motion cam retracts the center ring at a greater rate than that of the container such that the friction force between the center ring and the container helps to draw the can into the die. Therefore, a differential motion cam requires less force to be transmitted by the pusher. However, for both the constant motion and differential motion cams, the wear associated with the moving parts in the ram requires maintenance and equipment down-time at regular intervals.
The center ring floats with respect to the stationary die (in conventional motion tooling) because of clearance between the upper ram and its supports. A small amount of float in some circumstances may be beneficial to enable the tooling to accept cans that are slightly misaligned with respect to the center ring and to accommodate container ovality or variations in container wall thickness, which can be a result of the wall ironing process or prior necking stage. However, in either constant motion or differential motion cams, the float between the center ring and the die typically may be up to 0.012 (twelve thousandths) inch (0.305 mm) maximum amplitude in part because the clearances between the ram and its supports. Such a large degree of float, which increases with the wear of the ram and its sliding support, may allow the center ring to pinch the container against the die and may cause disuniform thickness of the neck. Further, the motion of the center ring may cause it to contact the die and may transfer lubricant to the inside of the can, which has obvious detrimental consequences.